To update this thread I did exchange some emails and photos with Anthony Vanderlinden about this weapon. He agreed 100% this was an early September 1944 build. He actually thinks the window of these being constructed may be as small as two weeks. He did include some of the following information including that he has a new book out and that there is a photo of another just like it on page 334-1 of his new book.
"Note that all of them were proofed by Louis Couchant, he was proofing these at FN. Some may misunderstand that the proofhouse had already re-opened, in fact the Liege proofhouse had a satelite office at FN where Couchant was busy proofing pistols.
These early postliberation pistols are rare. Congrats!